Apparatus for detecting the position of a rotor of an electric motor and related method

ABSTRACT

An apparatus detects the position of a rotor of an electric motor having three phases and a plurality of windings. The apparatus includes circuitry configured to connect at least two of said windings between first and second reference voltages according to a first current path disconnect said at least two windings, and allow the current stored in said two windings to be discharged through a second current path. The apparatus comprises a measuring circuit configured to measure the time period between the starting instant of storing the current in the two windings and the final instant of discharging the two windings and a rotor detector configured to detect the rotor position based at least in part on the measured time period.

BACKGROUND

1. Technical Field

The present disclosure relates to an apparatus for detecting theposition of a rotor of an electric motor, in particular a brushlessmotor, and to the related method.

The present disclosure permits to optimize the procedure of detecting oridentifying the position of the rotor, in particular in sensorlessbrushless motors typically used as spindle motors in hard disks, CDs,DVDs, etc.

2. Description of the Related Art

The identification procedure, normally called “inductive sense”, takesadvantage of the different response of the current in the motor windingwith respect to a voltage pulse applied to the ends of the windings ofthe motor itself. The knowledge of the rotor position permits tooptimize the motor startup procedure and is therefore a very importantfactor. The procedures of identifying the rotor position shouldtherefore be characterized by high performances in terms of precisionand insensitivity to disturbances.

Indeed an error in detecting the rotor position results in an impreciseexcitation sequence of the stator phases and in a consequent efficiencyreduction or, in the worst case, in a failure of the motor startupprocedure (loss of synchronism).

The methods of detecting the rotor position (inductive sense) of knownart only ensure good performances under conditions of a stopped rotor orwhile rotating at very low speed, i.e., under all those conditions inwhich the effect of the back electromotive force (BEMF) may beconsidered negligible.

Many startup techniques exist, some completely executed in open loop(startup in open loop) while in others, a detection of the rotor isindirectly used for generating the driving sequence, i.e., in closedloop (startup in closed loop).

The methods commonly used for detecting the position of the rotor atvery low speeds (or stopped) are based on the analysis of the current inthe windings upon a voltage pulse applied to the windings themselves.The principle is based on the phenomenon of magnetic saturation which isaffected by the position of the permanent rotor magnet and whichmodifies in turn the profile of the current. Therefore, by analyzing thecurrent, i.e., by analyzing the rise time Tr or the fall time Tf uponthe application of a voltage step, the position of the rotor may beunambiguously determined, and therefore the stator phases may be excitedto generate a torque of suitable value. The sequence of “identificationstep” (inductive sense) and “excitement step” (torque generation)permits to take the rotor to a speed such as to make the backelectromotive force (BEMF) detectable. When BEMF reaches sufficientlyhigh values, the startup procedure is considered concluded. Since then,the rotor position is extrapolated by directly analyzing the BEMF signaland no longer from the inductive sense procedure.

The inductive sense procedures are based on a comparison of certainparameters, typically the duration to reach a fixed current threshold,i.e., the rise time Tr, or the duration required to discharge thecurrent circulating in the motor windings, i.e., the fall time Tf. Inthese cases the result is affected by the errors made in the individualmeasurements, since it is based on the comparison of variousmeasurements made in sequence.

U.S. Pat. No. 6,841,903 describes a method for detecting the position ofa rotor in a DC motor having N phases and a plurality of windings,comprising the steps of: connecting at least two of the windings betweenfirst and second prefixed voltages according to a first current pathover a prefixed time; permitting the current stored in the two windingsto be discharged by means of a second current path; comparing thevoltage at the ends of one of the two windings with a third prefixedvoltage and supplying a control signal when the voltage has an absolutevalue lower than a third prefixed voltage; performing theabove-indicated steps for each of the pairs of motor windings; detectingthe rotor position according to the control signals obtained.

The typical problem of an inductive sense procedure is that of theprecision in the case wherein the motor moves, i.e., when the backelectromotive force (BEMF) is not null or at least negligible. Underthese conditions, the current pulses are modified by the presence ofBEMF, thus generating an error when detecting the rotor position.Therefore, in the known art systems, a maximum limit of BEMF (andtherefore of speed) exists, which may not be overcome if the desiredprecision and reliability in detecting the position of the rotor is tobe ensured.

BRIEF SUMMARY

One embodiment of the present disclosure is an apparatus for detectingthe position of a rotor of an electric motor, in particular a brushlessmotor, which overcomes the drawbacks of known apparatuses. With theapparatus in accordance with the disclosure, the problem of the positionerror generated by BEMF is minimized, thus ensuring the precision of theestimation of the rotor position for higher rotation speeds with respectto this which may be obtained, by using the known art inductive sensemethods.

One embodiment of the present disclosure is an apparatus for detectingthe position of a rotor of an electric motor, said electric motor havingthree phases, said apparatus comprising a plurality of windings andmeans for connecting at least two of said windings between first andsecond reference voltages according to a first current path, means fordisconnecting said at least two windings and means for allowing thecurrent stored in said two windings to be discharged through a secondcurrent path, characterized in that it comprises means adapted tomeasure the time period between the start instant of storing the currentin the two windings and the final instant of discharging the current inthe two windings, means for performing the aforesaid operations for eachpair of motor windings, means for detecting the rotor position accordingto said measured time periods.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The features and advantages of the present disclosure will becomeapparent from the following detailed description of practicalembodiments thereof, shown by way of non-limiting examples in theaccompanying drawings, in which:

FIG. 1 shows the partial diagram of a driving circuit of a brushlessmotor with an apparatus for detecting the position of the rotor of thebrushless motor according to a first embodiment of the presentdisclosure;

FIG. 2 shows the time diagrams of the rise and fall time of a currentpulse I which circulates in a pair of motor windings;

FIG. 3 shows the time diagrams of the rise and fall time of the currentpulse I obtained when Von=VCC and Voff=−(VCC+2 Vbe) and when BEMF=0;

FIG. 4 shows the time diagrams of the rise and fall time of the currentpulse I obtained when Von=VCC and Voff=−(VCC+2 Vbe) and when BEMF>0;

FIG. 5 shows the time diagrams of the differences of rise time periodsTr, fall time periods Tf, and sum periods Tr+Tf when BEMF=0 and thosewhen BEMF is other than zero, referred to the current pulse I as afunction of the BEMF when Von=VCC and Voff=−(VCC+2 Vbe);

FIG. 6 shows the time diagrams of the rise and fall time of the currentpulse I when BEMF=0 and with voltages Von and/or Voff being modulated toobtain Tr=Tf;

FIG. 7 shows the time diagrams of the rise and fall time of the currentpulse I when BEMF>0 and with voltages Von and/or Voff modulated toobtain Tr=Tf;

FIG. 8 shows the time diagrams of the differences of rise time periodTr, fall time periods Tf, and sum periods Tr+Tf when BEMF=0 and thosewhen BEMF is other than zero referred to the current pulse I as afunction of the BEMF and with voltages Von and/or Voff modulated toobtain Tr=Tf;

FIG. 9 is a block diagram of a controller according to one embodiment ofthe present disclosure;

FIG. 10 shows the time diagrams of rise time Tr, fall time Tf, and sumTr+Tf of a current impulse for a pair of windings at saturation and apair of windings not at saturation when Von=VCC and Voff=−(VCC+2 Vbe)and BEMF=0 determined by means of an apparatus for detecting theposition of a rotor of a brushless motor in accordance with a secondembodiment of the present disclosure;

FIG. 11 shows the time diagrams of rise time Tr, fall time Tf and sumTr+Tf of a current impulse for a pair of windings at saturation and apair of windings not at saturation when Von=VCC and Voff=−(VCC+2 Vbe)and BEMF>0 determined by means of an apparatus for detecting theposition of a rotor of a brushless motor in accordance with the secondembodiment of the present disclosure;

FIG. 12 shows the time diagrams of rise time Tr, fall time Tf and sumTr+Tf of a current impulse for a pair of windings at saturation and apair of windings not at saturation when Von=VCC and Voff=−(VCC+2 Vbe)and BEMF<0 determined by means of an apparatus for detecting theposition of a rotor of a brushless motor in accordance with the secondembodiment of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 shows an apparatus for detecting the position of a rotor of anelectric motor, in particular a brushless motor SM, according to anembodiment of the present disclosure; the electric motor is of theN-phase type, preferably having three phases. The apparatus comprisesthe half-bridges S1, S2 and S3 for controlling the brushless motor SMcomprising a rotor; FIG. 1 shows the stator of the electric motor SMwith the three star-connected phases U, V and W which include windingsAV1, AV2, and AV3, respectively. Each of the half-bridges consists of a“high side” transistor and a low side transistor, each with theassociated recirculating diode, indicated respectively by MHU, MLU andDHU, DLU for the half-bridge S1, and MHV, MLV and DHV, DLV for thehalf-bridge S2 and MHW, MLW and DHW, DLW for the half-bridge S3. Thehigh-side transistors have their drain terminal connected to thepositive power supply voltage terminal VCC while the low-sidetransistors have the source terminal connected to each other and toground GND, preferably by means of a resistor Rs. The source terminal ofeach high-side transistor is connected to the drain of each low-sidetransistor, the connection point of the half-bridge S1 corresponds tothe phase U and is connected to a terminal of the winding AV1, theconnection point of the half-bridge S2 corresponds to the phase V and isconnected to a terminal of the winding AV2, and the connection point ofthe half-bridge S3 corresponds to the phase W and is connected to aterminal of winding AV3. The other terminals of the windings AV1, AV2and AV3 are connected to one another. These windings diagrammaticallydepict a 3-phase spindle motor SM.

The gate terminals of the transistors of the half-bridges S1-S3 areconnected to driving circuits DriverU, DriverV, DriverW, respectively,which are configured to suitably power the phases U, V and W of themotor SM. The driving circuits DriverU, DriverV, DriverW are controlledby a control device MP. The voltages of back electromotive force (orBEMF) BemfU, BemfV and BemfW are at the terminals of the windings AV1,AV2 and AV3.

The positive input terminal of a comparator CompTr, a negative inputterminal of which a threshold voltage Ith is applied, is connected toresistor Rs. The output OutTr of comparator CompTr is connected to thecontrol circuit MP.

The positive input terminal of a comparator CompTf is connected, by aselector Sel comprising of three switches, to the three phases U, V, Wof the motor SM while the negative input terminal is connected to thepower supply voltage terminal VCC. The output OutTf of comparator CompTfis connected to the control circuit Mp, which also provides the commandsSw-U, Sw-V and Sw-W to selector Sel.

The apparatus in FIG. 1 uses the sensing resistor Rs for measuring thecurrent circulating in the windings during the excitation step, i.e.,for calculating the rise time Tr, while it uses the voltage drop at theterminals of the current recirculating diodes, connected in parallel tothe transistors of the half-bridges S1-S3, for measuring the dischargeor fall time Tf. However other methods for measuring the time periods Trand Tf are applicable, the apparatus in the figure is therefore to beconsidered merely exemplary.

If we consider, for example, the excitation of the phases U and V,indicated as U-V, the control circuit MP conveniently commands thecircuits DriverU and DriverV. In particular, circuit DriverU providesfor turning on the transistor MHU of the half-bridge S1 while circuitDriverV provides for turning on the transistor MLV of the half-bridgeS2. The current I will circulate through the windings AV1 and AV2 andmay be measured by means of the sensing resistor Rs. For the whole timethe voltage at the ends of resistor Rs, corresponding to the current Icirculating in the windings, is below the set threshold Ith, the outputOutTr of the comparator CompTr will be at the logic state “0”. As soonas the voltage across the resistor Rs reaches the set threshold Ith theoutput OutTr of the comparator CompTr will be brought to the logic level“1”. Upon reaching the voltage threshold Ith, the power stage S1-S3 isimmediately turned off (condition of high impedance) by means of thecontrol circuit MP and the process of discharging current I starts.

As soon as the power stage S1-S3 is placed under conditions of highimpedance, the current at the terminals of the resistor Rs inverts thedirection by taking back the output of the comparator CompTr to thelogic state “0”. The duration of the time period Tr is given by the timeelapsing from the moment T0 when the phase excitation is started and themoment T1 when the signal OutTr takes the logic level “1”, as shown inFIG. 2.

In the instant T1 when the voltage at the ends of resistor Rs(corresponding to the current flowing through the windings) reaches thethreshold Ith, the power stage is forced into high impedance and thedischarging process of current I starts. The current will circulate inresistor Rs in the opposite direction with respect to what occurredduring the excitation step, and will circulate through two diodes: onediode connected in parallel to a low-side transistor and one diodeconnected in parallel to a high-side transistor of the power stageS1-S3. Considering the excitation of the phases U-V, for example, theresult will be that, for the entire duration of the current discharge,i.e., over the time period Tf, diode DHV and diode DLU will beconducting. During the time period Tf, the voltage of phase V will takea value higher than the power supply voltage VCC while the voltage ofphase U will take a value lower than the ground voltage GND due to thevoltage drop at the ends of the conducting diodes. By using a comparatorCompTf the condition in which the voltage in phase V is greater than VCC(V>VCC) may be detected, accordingly the instant of time T2 may bedetected and the duration of the discharge time Tf given by T2-T1 may bemeasured, as shown in FIG. 2. Again in FIG. 2, the current impulse Icirculating in phase UV of motor SM may be seen.

As the excited phases change, obviously the diodes involved during thecurrent discharge step change. Accordingly, the selector SEL allows thecomparator CompTf to be connected to the correct terminal. Inparticular:

-   -   during the excitation of phase U-V, the selector SEL connects        the comparator CompTf to the terminal V;    -   during the excitation of phase W-V, the selector SEL connects        the comparator CompTf to the terminal V;    -   during the excitation of phase W-U, the selector SEL connects        the comparator CompTf to the terminal U;    -   during the excitation of phase V-U, the selector SEL connects        the comparator CompTf to the terminal U;    -   during the excitation of phase V-W, the selector SEL connects        the comparator CompTf to the terminal W;    -   during the excitation of phase U-W, the selector SEL connects        the comparator CompTf to the terminal W.

The determination of the rotor position is based on that the windingimpedance depends on the rotor position. For this reason and inparticular for a motor having three phases U, V and W, current I issequentially passed trough the following pairs of phases U-V, U-W, V-W,V-U, W-U, W-V, and the time period Ttot is measured, for each pair ofphases, where Ttot=Tr+Tf and the control circuit MP performs thesuitable calculations for determining the rotor position.

The advantage of the apparatus shown in FIG. 1 is in that each variationin the duration of the time period Tr by the BEMF is compensated by thevariation of the time period Tf and vice versa. That is, if BEMF has apolarity such as to reduce the rise time Tr of a current impulse I,likewise the fall time Tf of the same impulse will tend to increase andvice versa. Therefore, by measuring the total duration Ttot of thecurrent impulse Ttot=Tr+Tf an automatic compensation is obtained for theeffect introduced by BEMF.

The above-described method does not always permit a significant increaseof the performances because the charge time and discharge time are nottypically well equalized and therefore the variations introduced by BEMFare different in percentage. The compensation introduced by using thesum of rise time Tr and fall time Tf is thus not always optimal. Thedifferent duration of Tr and Tf is due to the load type with non-linearcharacteristic and to the different voltage which is typically used whencharging (charge voltage Von) and discharging (discharge voltage Voff)the current I.

The charging step is indeed typically obtained by applying the powersupply voltage VCC (Von=VCC) to the pair of windings involved, e.g.,windings AV1 and AV2, while the discharging step is typically obtainedby forcing the pair of windings involved (e.g., AV1 and AV2 again) intohigh impedance, i.e., by forcing a voltage Voff=−(VCC+2 Vbe) where Vbeis the voltage at the ends of a current recirculating diode connected inparallel to each power transistor; the charge voltage Von and thedischarge voltage Voff may take other values. In these cases, thedifference between the charge and discharge voltages is not negligibleand therefore the modulations of Tr and Tf due to the BEMF differentlyact in percentage. FIG. 3 indicates an example of current impulse Iobtained when Von=VCC and Voff=−(VCC+2 Vbe) in which the charge anddischarge of the current was considered linear.

FIG. 3 depicts the current impulse in the absence of BEMF characterizedby a rise time Tr1 and by a fall time Tf1 (when Tr1>Tf1) while FIG. 4depicts the same current impulse I under conditions of BEMF>0characterized by a rise time Tr1′ and by a fall time Tf1′ (whenTr1′>>Tf1′). FIG. 4 shows how the difference between Tr1 and Tr1′ (Trerror) is greater than the difference between Tr1+Tf1 and Tr1′+Tf1′(Tr+Tf error). The BEMF effects are therefore decreased by using thequantity Tr+Tf as a computation parameter instead of the simple Tr orTf.

FIG. 5 depicts the graph with the differences pattern of rise timeperiod Tr, fall time period Tf, and sum Tr+Tf when BEMF=0 and those whenBEMF is other than zero, according to the BEMF, if Von=VCC andVoff=−(VCC+2 Vbe). In the example, for simplicity, it is considered thecase of function I/t of the linear load having a current charge timeTr=100 us under conditions of BEMF=0. From the graph, the singlevariation of Tr and Tf, indicated as Tr_difference and Tf_difference, isnoted to be rather high whereas the variation of the sum Tr+Tf,indicated as Tr+Tf_difference, is smaller.

Alternatively the charge voltage Von and the discharge voltage Voff maybe the same according to the absolute value. In such a case, Von=VCC andVoff=−VCC. Thereby, better performance is obtained as compared toVon=VCC and Voff=−(VCC+2 Vbe), but the greater the function I/t of theload departs from linearity, the greater is the error, i.e., it is worsefor high currents.

Again alternatively the charge voltage Von and discharge voltage Voffapplied to a pair of windings of the motor SM may be modified ad hoc. Tominimize the error introduced by BEMF, the charge time Tr and dischargetime Tf may be made as equal as possible (in the absence of BEMF),thereby the variations of Tr and Tf by the BEMF will be better balanced.By applying an equal charge and discharge voltages, the discharge timeTf is always faster than the charge time Tr (when BEMF=0) andconveniently modulating the charge and/or discharge voltage enables oneto obtain Tr=Tf under conditions of BEMF=0. If the power stage mayfunction in linear AB-class, the condition is easily obtainable byactuating an initial calibration in which the condition Tr=Tf whenBEMF=0 is forced by acting on the charge and/or discharge voltage. Oncethe charge and/or discharge voltage is found, which permits to obtainTr=Tf, the measurements with BEMF>0 may be carried out, where theeffects of BEMF on Tr+Tf will be minimized. In other words, thevariations of Tr and Tf by the BEMF will be such as to minimize theparameter Tr+Tf used in computation.

FIG. 6 indicates an example of current impulse to which the compensationmethod of the BEMF has been applied, i.e., with the voltages Von and/orVoff modulated to obtain Tr=Tf by assuming that the charge and dischargeof the current is linear. FIG. 6 depicts the current impulse in theabsence of BEMF, characterized by a rise time Tr2 and by a fall time Tf2when Tr2=Tf2; FIG. 7 depicts the same current impulse under conditionsof BEMF>0 characterized by a rise time Tr2′ and by a fall time Tf2′(Tr2′>Tf2′). FIG. 7 shows how the difference between Tr2 and Tr2′(Tr2_error) is much greater than the difference between Tr2+Tf2 andTr2′+Tf2′ (Tr2+Tf2_error). The effects of BEMF are thus significantlydecreased by using the quantity Tr+Tf as a computation parameter insteadof the simple Tr or Tf if a voltage Von and/or Voff is used such as tohave Tr=Tf under conditions of BEMF=0.

FIG. 8 depicts the graph with the differences pattern of the rise periodTr, fall period Tf, and sum Tr+Tf when BEMF=0 and those when BEMF isother than zero, according to the BEMF, if Von and/or Voff are modulatedto obtain Tr=Tf. In the example, it is considered the case of functioni/t of the linear load having a current charge time Tr=100 microsecondsunder conditions of BEMF=0. From the graph, the single variation of Tr2and Tf2, indicated as Tr2_difference and Tf2_difference, is noted to berather high whereas the variation of the sum Tr2+Tf2, indicated asTr2+Tf2_difference, is smaller.

In accordance with one embodiment of the disclosure shown in FIG. 9, thecontroller MP includes various functional modules that can beimplemented by software/firmware modules of a processor, hardwarecircuits, and/or a combination of hardware and software. In particular,the controller MP includes a timer module 10 configured to determine thecharging time Tr based on the signal OutTr produced by the comparatorCompTr, the discharging time Tf based on the signal OutTf produced bythe comparator CompTf, and the sum of the charge and discharge times.The controller MP also includes a driver controller 12 configured tocontrol the drivers DriverU, DriverV, and DriverW, of the drive stagesS, S2, S3, respectively, a selector controller 14 configured to controlthe selector Sel, and a position detector 16 configured to determine theposition of the rotor of the brushless motor based on the time periodsgiven by the addition of each rise time Tr and the fall time Tf whenBEMF=0 The controller MP also comprises a correction calculator 18configured to calculate a corrective factor K to be applied to Tf (orTr), which permits to obtain the condition Tr=KTf (or Ktr=Tf). Once thecorrective factor K has been determined and provided to the timer module10, the timer module can determine and provide Tr+KTf (or Ktr+Tf) as acomputation parameter to the position detector 16 for estimating therotor position. Indeed, in most cases, the power stage is of the ON/OFFtype (i.e., belongs to class D) and therefore the charge or dischargevoltage may not be modulated as desired. Of course, one skilled in theart will recognize that the modules 10-18 could be implemented byseparate elements or any and all of the modules could be combined intoone or more combined modules or further divided into more modules thatperform more specific functions.

By way of example, FIG. 10 shows the two current impulses I which crossthe windings of motor SM at maximum saturation conditions, for examplein the phase U-V indicated by COIL_1, and at the minimum saturationconditions, in the phase V-U indicated by COIL_2, when BEMF=0, with therotor aligned (or close to alignment) to the phase U-V. FIG. 10considers a real load (not linearized) in which the inductance valuevaries by 10% due to the saturations introduced by the position of therotor. In FIG. 10, the characteristic times may be extracted, whichrelate to the current impulse which passes through the winding withmaximum saturation in the phase U-V, i.e., rise time Tr1, fall time Tf1,and sum Tr1+Tf1 thereof.

When Tr1=102 microseconds, Tf1=41.5 microseconds, Tr1+Tf1=143.5microseconds, the result is K1=Tr1/Tf1=2.48=K and Ttot=Tr1+K1*Tf1=204microseconds in the phase U-V indicated by COIL_1. Therefore, thecorrective factor to be applied to Tf1 (in our example) is:K=K1=Tr1/Tf1=2.48.

As described above, again in FIG. 10, rise time Tr2, fall time Tf2, andtheir sum Tr2+Tf2 may be extracted, related to the current which passesthrough the winding with minimum saturation in the phase V-U and thecorrective factor K2 is calculated: the result is Tr2=112 microseconds,Tf2=45 microseconds, Tr2+Tf2=157 microseconds and K2=Tr2/Tf2=2.48=K andTtot=Tr2+K2*Tf2=224 microseconds. The corrective factor to be applied toTf2 (in our example) is: K=K2=Tr2/Tf2=2.48. The corrective factor isequal to that previously found, whereby it is sufficient to calculate itonly once according to the current which passes through the winding inany one of the phases U-V, U-W, V-W, V-U, W-U, W-V under conditions ofBEMF=0.

If BEMF is other than zero (i.e., the motor SM is moving), thecircumstance is modified as shown in FIG. 11, where the voltage BEMF isof a sign such as to oppose the power supply voltage VCC when excitingthe saturated phase U-V and accordingly of sign such as to be added tothe power supply voltage when exciting the non-saturated phase V-U. Theeffect will be a slow-down of the rise edge of the current circulatingin the phase U-V, i.e., a greater rise time Tr, and a greater speed ofthe rise edge of the current circulating in the phase V-U, i.e., asmaller rise time Tr. The subject is reversed with regards to fall timesTf.

When BEMF=+300 mV in the phase U-V (BEMF=−300 mV in the phase V-U), theresult is Tr1′=114 microseconds, Tf1′=40 microseconds, Tr1′+Tf1′=154microseconds and using K=2.48, the result is Ttot=Tr1′+K*Tf1′=213microseconds in the phase U-V indicated by COIL_1 and Tr2′=101microseconds, Tf2′=47.7 microseconds, Tr2+Tf2=148.7 microseconds andusing K=2.48, the result is Ttot=Tr2+K*Tf2=217 microseconds in the phaseV-U indicated by COIL_2.

Thus, the presence of BEMF modifies the current impulses by making therise edge of the current circulating in the windings at saturation inthe phase U-V slower than the rise edge of the current circulating inthe windings not at saturation in the phase V-U. An inductive senseprocedure of known art which uses the parameter Tr as a computationparameter is thus not capable of ensuring the correct result in thepresence of BEMF.

If BEMF is other than zero (the motor moves) but has an oppositepolarity to that previously described for FIG. 11, i.e., if the voltageBEMF is of sign such as to be added to the power supply voltage whenexciting the saturated phase U-V and accordingly of sign such as to besubtracted from the power supply voltage when exciting the non-saturatedphase V-U (FIG. 12). The effect will be greater speed of the rise edgeof the current circulating in the phase U-V, i.e., a smaller rise timeTr, and a slow down of the rise edge of the current circulating in thephase V-U, i.e., a greater rise time Tr. The subject is reversed withregards to fall times Tf.

When the BEMF=−300 mV in the phase U-V (BEMF=+300 mV in the phase V-U),the result is Tr1″=92 microseconds, Tf1″=43 microseconds, Tr1″+Tf1”=135microseconds and using K=2.48, the result is Ttot=Tr1″+K*Tf1“=198microseconds in the phase U-V indicated by COIL_1 and Tr2″=125microseconds, Tf2″=44 microseconds, Tr2″+Tf2″=169 microseconds and usingK=2.48, the result is Ttot=Tr2″+K*Tf2″=234 microseconds in the phase V-Uindicated by COIL_2.

By analyzing the data extrapolated from FIG. 10, Tr1<Tr2, Tf1<Tf2,(Tr1+Tf1)<(Tr2+Tf2) and (Tr1+KTf1)<(Tr2+KTf2), i.e., in the absence ofBEMF all parameters are consistent and all parameters may be used in thecomputation of the rotor position.

By analyzing the data extrapolated from FIG. 11, Tr1′>Tr2′ is inferred,which is not consistent with the expectations, Tf1′<Tf2′ consistentlywith the expectations, (Tr1′+Tf1′)>(Tr2′+Tf2′) which is not consistentwith the expectations, (Tr1'+KTf1′)<(Tr2′+KTf2′) consistently with theexpectations. The effects of BEMF affect certain parameters which arethus not usable in the computation of the rotor position. Also even theparameter Tr+Tf is not capable of ensuring the expected result asinstead the parameter Tr+KTf, ensures the expected result.

By analyzing the data extrapolated from FIG. 12, it is obtainedTr1″<Tr2″ consistently with the expectations, Tf1″˜=Tf2″ which is notconsistent with the expectations, (Tr1″+Tf1″)<(Tr2″+Tf2″) consistentlywith the expectations and (Tr1″+KTf1″)<(Tr2″+KTf2″) consistently withthe expectations. Even in this case, the effects of BEMF affect certainparameters which are thus not usable in the computation of the rotorposition. Thus, from the data extrapolated from FIGS. 10-12, the onlyparameter which is always consistent with the expectations is(Tr1″+KTf1″)<(Tr2″+KTf2″). A similar result would have been obtained bymodulating the discharge voltage according to the disclosure of thesecond embodiment but this would have required a power stage notoperating in class D.

The use of the present disclosure thus permits to extend thefunctionality of the inductive sense procedure to rotation speeds whichare higher than those usable in the known art systems.

In all the above-described embodiments of the disclosure and in theirvariants, the parameters used for estimating the rotor position are thetimes to reach certain current thresholds (Tr, Tf and others obtained byprocessing them). However, this is only one of the methods forparameterizing the current impulses which characterize the inductivesense procedure and which permit to implement the present disclosure. Ingeneral, any other parameter having a relationship with the currentcharge (therefore not necessarily Tr) and any other parameter having arelationship with the current discharge (therefore not necessarily Tf)may be used for implementing the present disclosure. Thereby, thepresent disclosure allows a “general parameter” related to the currentcharge to be at most equalized to the “general parameter” related to thecurrent discharge (when BEMF=0), independently from the selectedparameter, and both the parameters to be used in the computation fordetermining the rotor position.

In particular the present disclosure concerns the techniques ofdetecting the rotor position for carrying out the closed loop startup.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. An apparatus, comprising; an electric motor having a plurality of windings and a rotor; a plurality of drive stages configured to produce a first current through a first pair of said windings and to discharge the first current; a measuring circuit configured to measure a first time period between a starting instant of producing the first current in the first pair of windings and a final instant of discharging the first current from the first pair of windings; and a rotor detector configured to detect a position of the rotor based at least in part on said measured first time period.
 2. An apparatus according to claim 1, wherein said measuring circuit includes: a charge measurement circuit configured to measure a charge time period until the first current reaches a threshold value; and a discharge measurement circuit configured to measure a discharge time period during which the first current reaches a null value from the threshold value, said first time period corresponding to a sum of the charge time period and the discharge time period.
 3. An apparatus according to the claim 2, wherein said drive stages are configured to produce the first current by connecting the first pair of windings between a power supply voltage and ground.
 4. An apparatus according to claim 3, wherein the drive stages are configured to produce the first current by applying a charge voltage, equal to the power supply voltage, to the first pair of windings, and are configured to discharge the first current by applying a discharge voltage, equal to the power supply voltage but with opposite polarity, to said first pair of windings.
 5. An apparatus according to claim 2, comprising a drive stage controller configured to cause the drive stages to apply voltages to said first pair of windings such that the charge time period is equal to the discharge time period .
 6. An apparatus according to claim 2, comprising a correction factor calculator configured to determine and apply a corrective factor to the charge time period or to the discharge time period such that either the charge time period equals said factor K multiplied by the discharge time period, or the discharge time period equals said factor K multiplied by the charge time period.
 7. An apparatus according to claim 2, wherein each of the drive stages includes a half-bridge that includes first and second transistors and first and second current recirculating diodes, respectively, the apparatus comprising a drive stage control configured to cause the drive stages to apply a voltage equal to a power supply voltage to said first pair of windings while producing said first current, and configured to cause the drive stages to apply to the first pair of windings a voltage equal to a sum of the power supply voltage and twice a voltage at terminals of the recirculating diodes while discharging the first pair of windings.
 8. An apparatus according to claim 2, wherein the drive stages respectively include first, second, and third half-bridges, each half-bridge including first and second transistors connected to each other at a central point of the half-bridge, wherein the measuring circuit includes: a first comparator having a first input, a second input, and an output, the first input being coupled to a power supply voltage; a selector configured to selectively connect the second input of the first comparator to the central points of said half-bridges, respectively; a second comparator configured to compare a voltage corresponding to the first current with a reference voltage; and a control circuit configured to receive outputs of said first and second comparators and determine the first time period as a function of the outputs of the comparators.
 9. An apparatus according to claim 1, wherein; the plurality of windings includes three windings; the plurality of drive stages includes first, second, and third drive stages configured to sequentially produce the first current and second, third, fourth, fifth, and sixth currents through said windings and to sequentially discharge the first, second, third, fourth, fifth, and sixth currents; the first current is produced in a first direction through the first pair of the windings, the second current is produced in the first direction through a second pair of the windings, the third current is produced in the first direction through a third pair of the windings, the fourth current is produced in a second direction through the first pair of the windings, the fifth current is produced in the second direction through the second pair of the windings, and the sixth current is produced in the second direction through the third pair of the windings; the measuring circuit configured to measure a second time period between a starting instant of producing the second current and a final instant of discharging the second current from the second pair of windings, measure a third time period between a starting instant of producing the third current and a final instant of discharging the third current from the third pair of windings, measure a fourth time period between a starting instant of producing the fourth current and a final instant of discharging the fourth current from the first pair of windings, measure a fifth time period between a starting instant of producing the fifth current and a final instant of discharging the fifth current from the second pair of windings, measure a sixth time period between a starting instant of producing the sixth current and a final instant of discharging the sixth current from the third pair of windings; and the rotor detector is configured to detect positions of the rotor based on said measured first, second, third, fourth, fifth, and sixth time periods.
 10. A method for detecting a position of a rotor of an electric motor having a plurality of windings, comprising: producing a first current through a first pair of the windings by connecting the first pair of windings between first and second reference voltages according to a first current path; discharging said first pair of windings through a second current path; measuring a first time period between a starting instant of connecting the first pair of windings between the first and second reference voltages and a final instant of discharging the first pair of windings; detecting the position of said rotor based at least in part on said measured first time period.
 11. A method according to claim 10, wherein measuring the first time period includes: measuring a charge time period until the first current reaches a threshold value; and measuring a discharge time period during which the first current reaches a null value from the threshold value, the first time period corresponding to a sum of the charge time period and the discharge time period.
 12. A method according to claim 10, wherein producing the first current includes applying a charge voltage to said first pair of windings and discharging the first current includes applying a discharge voltage such that the charge time period is equal to the discharge time period.
 13. A method according to claim 10, comprising determining and applying a corrective factor to the charge time period or to the discharge time period such that either the charge time period equals said factor K multiplied by the discharge time period, or the discharge time period equals said factor K multiplied by the charge time period.
 14. A method according to claim 10, wherein producing the first current includes applying a charge voltage, equal to a power supply voltage, to the first pair of windings, and discharging the first current includes applying a discharge voltage, equal to the power supply voltage but with opposite polarity, to said first pairs of windings.
 15. A method according to claim 10, comprising; producing a second current through a second pair of the windings by connecting the second pair of windings between the first and second reference voltages according to a third current path; discharging said second current through a fourth current path; measuring a second time period between a starting instant of producing the second current and a final instant of discharging the second current; producing a third current through a third pair of the windings by connecting the third pair of windings between the first and second reference voltages according to a fifth current path; discharging said third current through a sixth current path; measuring a third time period between a starting instant of producing the third current and a final instant of discharging the third current; producing a fourth current through the first pair of the windings by connecting the first pair of windings between the first and second reference voltages according to a seventh current path; discharging said fourth current through an eighth current path; measuring a fourth time period between a starting instant of producing the fourth current and a final instant of discharging the fourth current; producing a fifth current through the second pair of the windings by connecting the second pair of windings between the first and second reference voltages according to a ninth current path; discharging said fifth current through a tenth current path; measuring a fifth time period between a starting instant of producing the fifth current and a final instant of discharging the fifth current; producing a sixth current through the third pair of the windings by connecting the third pair of windings between the first and second reference voltages according to an eleventh current path; discharging said sixth current through a twelfth current path; and measuring a sixth time period between a starting instant of producing the sixth current and a final instant of discharging the sixth current, wherein detecting the position of said rotor includes detecting positions of the rotor based at least in part on said measured first, second, third, fourth, fifth, and sixth time period.
 16. An apparatus for detecting a position of a rotor of an electric motor having a plurality of windings, the apparatus comprising: a plurality of drive stages configured to produce a first current through a first pair of said windings and to discharge the first current; a measuring circuit configured to measure a first time period between a starting instant of producing the first current in the first pair of windings and a final instant of discharging the first current from the first pair of windings; and a rotor detector configured to detect a position of the rotor based at least in part on said measured first time period.
 17. An apparatus according to claim 16, wherein said measuring circuit includes: a charge measurement circuit configured to measure a charge time period until the first current reaches a threshold value; and a discharge measurement circuit configured to measure a discharge time period during which the first current reaches a null value from the threshold value, said first time period corresponding to a sum of the charge time period and the discharge time period.
 18. An apparatus according to the claim 17, wherein said drive stages are configured to produce the first current by connecting the first pair of windings between a power supply voltage and ground.
 19. An apparatus according to claim 18, wherein the drive stages are configured to produce the first current by applying a charge voltage, equal to the power supply voltage, to the first pair of windings, and are configured to discharge the first current by applying a discharge voltage, equal to the power supply voltage but with opposite polarity, to said first pair of windings.
 20. An apparatus according to claim 17, comprising a drive stage controller configured to cause the drive stages to apply voltages to said first pair of windings such that the charge time period is equal to the discharge time period.
 21. An apparatus according to claim 17, comprising a correction factor calculator configured to determine and apply a corrective factor to the charge time period or to the discharge time period such that either the charge time period equals said factor K multiplied by the discharge time period, or the discharge time period equals said factor K multiplied by the charge time period.
 22. An apparatus according to claim 17, wherein each of the drive stages includes a half-bridge that includes first and second transistors and first and second current recirculating diodes, respectively, the apparatus comprising a drive stage control configured to cause the drive stages to apply a voltage equal to a power supply voltage to said first pair of windings while producing said first current, and configured to cause the drive stages to apply to the first pair of windings a voltage equal to a sum of the power supply voltage and twice a voltage at terminals of the recirculating diodes while discharging the first pair of windings.
 23. An apparatus according to claim 17, wherein the drive stages respectively include first, second, and third half-bridges, each half-bridge including first and second transistors connected to each other at a central point of the half-bridge, wherein the measuring circuit includes: a first comparator having a first input, a second input, and an output, the first input being coupled to a power supply voltage; a selector configured to selectively connect the second input of the first comparator to the central points of said half-bridges, respectively; a second comparator configured to compare a voltage corresponding to the first current with a reference voltage; and a control circuit configured to receive outputs of said first and second comparators and determine the first time period as a function of the outputs of the comparators.
 24. An apparatus according to claim 16, wherein; the plurality of drive stages includes first, second, and third drive stages configured to sequentially produce the first current and second, third, fourth, fifth, and sixth currents through said windings and to sequentially discharge the first, second, third, fourth, fifth, and sixth currents; the first current is produced in a first direction through the first pair of the windings, the second current is produced in the first direction through a second pair of the windings, the third current is produced in the first direction through a third pair of the windings, the fourth current is produced in a second direction through the first pair of the windings, the fifth current is produced in the second direction through the second pair of the windings, and the sixth current is produced in the second direction through the third pair of the windings; the measuring circuit configured to measure a second time period between a starting instant of producing the second current and a final instant of discharging the second current from the second pair of windings, measure a third time period between a starting instant of producing the third current and a final instant of discharging the third current from the third pair of windings, measure a fourth time period between a starting instant of producing the fourth current and a final instant of discharging the fourth current from the first pair of windings, measure a fifth time period between a starting instant of producing the fifth current and a final instant of discharging the fifth current from the second pair of windings, measure a sixth time period between a starting instant of producing the sixth current and a final instant of discharging the sixth current from the third pair of windings; and the rotor detector is configured to detect positions of the rotor based on said measured first, second, third, fourth, fifth, and sixth time periods. 